As thermodynamic media expand isentropically through a power turbine in a Rankine cycle system, the vapor quality varies for any vapor whose saturation curve across the pressure range traversed during that expansion is not parallel with the isentropic value along which the expansion occurs. When steam is the medium being expanded, this results in the vapor proceeding from a possible superheated region at high temperature and pressure, through the saturation range, and finally may enter a "wet" vapor condition as exhaust pressure is reached. It has become common practice to employ a "reheat" cycle to overcome difficulties resulting from this steam characteristic. The steam, after partial expansion along the turbine cycle, is extracted and returned to the boiler for reheating up to a new superheated condition for its now reduced pressure, and then returned to the turbine to continue further expansion. Excessive moisture in the steam ( i.e.--generally a vapor quality less than perhaps 88%) can cause loss of efficiency in the turbine and can cause blade damage and pitting due to moisture particle impact of the back sides of the blading.
Recent interest in use of hydrocarbon and fluorocarbon media in low-temperature turbine cycles (commonly known as Organic Rankine Cycles) has introduced use of media which frequently behave characteristically in a manner opposite to that of steam during expansion. Many of these turbine media expand isentropically along a curve of reverse slope to that of their saturation curves. As a result, such media may start at the beginning of their expansion in a wet or saturated condition, become progressively drier or superheated during expansion as they diverge from the saturation curve, and frequently arrive at final exhaust pressure in a superheated condition. Under these conditions, the superheat content of the vapor at exhaust may be lost as additional waste heat, substantially hotter than saturation temperature for the exhaust pressure, leaving both the superheat and the latent heat to be removed by condenser cooling water to effect condensation.
It has also been common steam turbine practice to provide means for extracting a portion of the expanding steam at various locations along the expansion process and to use the extracted steam to heat the returning feed water stream. This is known as the "regenerative" Rankine cycle. In the process, a portion of the heat content of the extracted steam is retained within the circulating turbine cycle that would otherwise have been lost as waste heat in the condenser. That heat energy loss prevention contributes increased thermodynamic efficiency to the total turbine cycle. However, the mass flow of the amount of steam extracted for this purpose becomes an amount that was never expanded all the way to the exhaust conditions, and therefore does not contribute all the output power that might have been available had it been expanded all the way to condenser pressure.
Also from the prior art is known an analogous technique for recovery of some exhaust superheat condition in the cycle of one of the reverse-slope media turbine cycles and cooling it via heat exchanger means with the boiler return feed stream before completing the condensation function closer to saturation temperature for the exhaust pressure. It thereby recovers much of what would have been waste superheat loss in the condenser by regenerative feed stream heating. This cycle is known as a "recuperative" cycle.
It is also known to take advantage of the characteristic reverse slope of the turbine media that dry on expansion (viz.--butane, isobutane, iso-pentane, and several of the fluorocarbons) by provision of one or more injectors located along the expansion route of the medium through the turbine at which it may become desirable to reduce developing superheat or drying out of the expanding turbine medium, by injecting a controlled amount of liquid phase turbine medium into the vapor stream passing through the turbine at that point. The mixture of the liquid injected with the vapor in transit creates a new thermodynamic state condition in the flowing fluid, desuperheated or wetter than the superheated condition it had reached just before the point of injection. Depending on the proportions of the mass flow of liquid injected to the mass flow of the vapor into which it is injected, the ensuing vapor quality of the mixture can be controlled to whatever level is preferred so that ensuing further expansion will result in arriving at final exhaust conditions with a lower superheat content for the pressure at which ultimate condensation of the exhaust is intended to occur. If the pressure range across which isentropic expansion occurs is great enough, or the slope is great enough to cause more rapid drying during expansion, two or more injection points along the expansion process may be desired to control moisture content of expanding vapor within preferred limits.
Final power output of the turbine is also related to the mass flow of turbine medium undergoing expansion through the turbine. As additional medium is injected to absorb evolving superheat, mass flow is also increased for the on-going expansion process beyond the point of injection, contributing an additional increment of output power to the turbine cycle. The higher the temperature at which injected medium is introduced to the turbine, the greater mass flow can be injected to effect desuperheating of the expanding medium and thereby further increase ensuing mass flow being expanded in remaining portions of the turbine cycle below the point of injection. U.S. Pat. No. 3,234,734 to Buss, et al. incorporated herein by reference teaches this concept. In a regenerative Rankine cycle, quantities of turbine medium in the feed stream return were progressively heated by medium extraction points along the turbine cycle which provided sources of liquid turbine medium at progressively higher temperatures along the feed stream return path. These sources were used to supply injection liquid phase medium to desuperheat the vapor flow at selected injection points along the turbine expansion cycle. In that teaching, the heat source elevating the feed stream temperature, by regenerative extraction of vapor from the turbine, originated from heat energy already within the expanding turbine medium within the turbine. That encumbered a loss of turbine mass flow to supply the vapor extraction (a characteristic of all regenerative Rankine cycles).
In U.S. Pat. No. 3,234,734, to J. R. Buss et al., preheating was accomplished by extraction of hot vapor from the turbine itself (as practiced in conventional regenerative turbine cycles), but in that process, heat energy content of the medium mass flow through the turbine was reduced and then replaced, to effect the benefits realized in superheat waste reduction.